The present invention relates to thermography apparatus and, in particular, to the conveying of sheets which are to be printed thermographically on both sides.
Thermography, or raised printing, is carried out by applying a thermography powder, such as a resin-based powder, onto a printed sheet while the ink thereon is still tacky. Sufficient heat is applied to melt the powder, whereupon the powder expands and thus imparts a "raised" effect to the printed areas.
One type of thermography apparatus which has heretofore been employed for carrying-out raised printing operations has included a powder-applying section through which the printed paper sheets are fed. In this section, resinous powder, e.g., a nylon resin, is applied from a hopper onto the printed surface of each sheet and adheres to the still-tacky ink. Thereafter, excess powder is sucked from the sheet by a vacuum pick-up head, leaving powder only on the ink. The sheets are then conveyed through a heat tunnel in which the sheets are powder are heated to the melting temperature of the powder. As the powder melts, it expands and thus "raises" the printing. Thereafter, the sheets are conveyed through a cooling tunnel wherein the raised print is solidified.
During that process, the sheet is conveyed along one or more conveyor belts through a powder-applying zone, a heating zone, and a cooling zone. The powder is applied to the top side of the sheet, while the underside thereof rests upon the conveyor belt. The conveyor belt is of a type capable of withstanding high temperatures, such as a stainless steel wire-mesh belt.
In some cases it is desirable to subject both sides of the sheet to a raised printing process. This is achieved, following the raised printing treatment of the top side of the sheet, by inverting the sheet and again passing the sheet through the thermography apparatus. As the sheet passes again through the heating zone, the resin previously applied to the side of the sheet now faces the conveyor belt and is melted in areas where it is contacted by the hot wires of the mesh belt. As a result of that contact and melting, there occurs a tendency for the resin to be smeared, thereby marring the aesthetic appearance of the product.
In an effort to alleviate that problem, it has therefore been proposed in Nadelson U.S. Pat. No. 3,526,207, to provide the conveyor belt with upstanding pointed pins upon which the sheets are supported. As a result, the areas of contact between the resin and belt are constituted by minutes points, whereby little, if any, readily detectible smearing of the resin will occur. The Nadelson patent discloses the provision of inserts which can be positioned within the conveyor belt links. The insert comprises a flat bar which can be slid into a lateral spaced formed between the wires. The bar includes a series of threaded holes which face upwardly after the bar has been inserted. The holes are adapted to receive the threaded ends of pointed pins by screwing the pins into the holes after the bar has been inserted into the belt. The pins are dimensioned to project above the belt to support the sheets.
Such a proposal involves certain technical drawbacks. For example, in order to properly modify a typical thermotype conveyor belt under the Nadelson proposal, it would be necessary to manually screw-in a larger number, perhaps hundreds, of pins, a very time-consuming process. Furthermore, since the pins will be subjected repeatedly to high and low temperature cycles, it is quite likely that the repeated thermal expansion and contraction of the pins and bars will, over a period of time, produce a loosening of the pins. Moreover, Nadelson does not appear to disclose any means for anchoring the bars against movement relative to the conveyor belt. Any such relative movement which occurs may magnify the smearing of the resin to such as extent that the smearing becomes more readily detectible.
It is, therefore, an object of the present invention to minimize or obviate shortcomings of the type discussed above.
A further object is to provide a sheet-supporting insert for a thermography conveyor belt which can be easily and rapidly installed.
A further object is to provide such an insert wherein the sheet contacting portions cannot become loosened.
An additional object is to provide such an insert which can be anchored against movement relative to the conveyor belt.